During his exile, Yawgmoth turned the Creeping Mold of Argoth into a virulent plague that ate away the elves there. He also kidnapped Elyssendril Lademmdrith and her healers, ordering the elves to pay ransom for their leader and their cure. When the elves paid, he delivered to them only sweetened water and 12 dead healers.
At the meeting in Halcyon it was mentioned that she was of the Daelic elves, and that she represented a vast confederation of woodland nations on the Domains. Since Argoth is not part of the Domains, this is a bit of a mystery.